Pinta daily PPA on Ubuntu 14.04. General rule of settings in Pinta is there are preserved during current Pinta session (from Pinta startup to Pinta close). For example click on Text and from font size select 20 and type some text. Now click on paintbrush and select width to 20 and draw something on canvas. Now click back to Text and you will see font size 20 has been preserved, also if clicked back to Paintbrush the width is still 20. So the logic is to preserve all of the settings in current session.
Now to the problem.
1. Click on Line.
2. Click to check Arrow 2.
3. Draw some line on canvas, but do not finalize it (don't press Enter).
4. Select Size=10, Angle=0 and Length=10.
5. Click on Text and type some text on canvas.
6. Click on Line and Arrow 2 and values for Size, Angle and Length looks like they have preserved.
7. Draw arrow on canvas and it is clear that above values are not respeced. Press Enter to finalize the line.
8. Draw another arrow on canvas and it is clear that now default values are respected.
So the problem is tool is changed and then back to Line, values are first time not working fine.
If I am not mistaken Andrew in GSoC-2014 has fixed this problem. But I suggest to leave this bug opened until his code is merged into official Pinta project.